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    SubjectRe: [PATCH] module interface improvement for kprobes
    On Fri, Aug 04, 2006 at 01:30:39PM -0500, David Smith wrote:
    > Why shouldn't I put a probe into a module other than at a symbol I can
    > find with kallsyms? For example, I'm interested when a particular
    > module hits an error condition that occurs. I don't want to probe how
    > many times the function gets called - just when the error condition
    > occurs.

    How do you find that offset? You'll probably mention the S-Word but
    we really want something that works with the latest kernel, not just
    the vendor trees.

    > With the existing interface, if I use kallsysms to find the value of a
    > symbol, the module can be unloaded between the time I use kallsyms and
    > register the kprobe. The patch I included fixes that race condition by
    > incrementing the module reference count.

    Yes, and that's a good thing. But the interface for doing it is wrong.
    You don't really want the users to do all that by itself. For the typical
    case of putting a probe at the usual points you want an interface that
    puts in the probe given a name and does the right thing for you. For example
    the interface I proposed in my last mail. Adding another field to struct
    kprobe to specify an offset into the symbol would be the logical extension
    of that.

    > Your example works for a very small number of symbols, but with a large
    > number it could take a long time to register the kprobes. Plus, that
    > would need to be done every time the kprobe was registered. With my
    > patch, the symbol lookup can be done once, then all those symbols can be
    > turned into offsets from the base address of the module.

    Registering a kprobe is everything but a fastpath, and you definitly should
    not have a lot of probes anyway. It's far more worthwhile to have a sane
    interface that the user can't get wrong then a small speedup in something
    that's not a fastpath. I think Rusty even has a paper or talk about why
    this is absolutely nessecary :)
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